Risk and Reward in Momentum Trading

Transcription

1 Risk and Reward in Momentum Trading Richard Martin Founding Partner, Longwood Credit Partners LLP Imperial College London University College London Copyright 2013 Richard Martin. All rights reserved

2 What momentum trading is Flippantly: Buy high and sell low What I actually mean by this is: Buy high, sell higher; Sell low, buy (back) lower Or: Buy what is going up, sell what is going down It works because... anchoring bias information leakage in some asset classes (short-term govt bonds, STIRs) Government policy 2

3 Is it really that simple? Buy what is going up; sell what is going down Momentum investing shouldn t work. Markets are supposed to be efficient... But it does work, prompting Eugene Fama to describe it as the premier anomaly. The team at London Business School mined their database of long-term stock returns all the way back to 1900 and found that buying the 20 best performers from the 100 biggest UK shares generated an average annual return of 14.1% (Jonathan Eley, FT, 8 Mar 2013) If momentum investing is so simple, why don t more people do it? One reason might be that its crudeness offends our intellectual sensibilities. Investing is supposed to involve a degree of judgment and intelligence, whereas momentum is a mechanical process involving little conscious thought. Anyone could do it. (ibid) 3

4 Specific conclusions of this talk The first moment (expected return) is a matter of empiricism/belief But the second and third moments of momentum trading returns are of technical interest Momentum trading returns are known to have positive skewness thus they hold onto profits, which is good also implies that the proportion of winning trades may be less than one-half this is true even if expected return is zero intriguing and useful that we can design skewness into a trading strategy How does this help us design systems? 4

5 Design principles General trading principle (not just momentum): Scale positions inversely to volatility thereby keeping bets evenly-sized or, strategy (trading) returns have roughly constant volatility as a result so, position signal vol : 2 t G / G / t t t t Signal will be related to previous price history: >0 in uptrend, <0 in downtrend essentially, all these operate by taking weighted average of previous returns we will do this using exponentially-weighted moving averages, though we don t have to Transaction costs as each time we change position we incur costs not for this seminar, but I have done quite a bit of work in this area 5

6 Moving averages A common device in momentum / technical analysis is the moving average Use exponentially-weighted (EMA) and compare either of: EMA1: spot minus moving average. When spot > EMA, uptrend or EMA2: fast minus slow example: USDJPY 6

7 filter weight Filter weights for EMA1 and EMA EMA1 (16) EMA2 (8:16) lag 7

8 SP1 momentum signals 3 Momentum signals are smoothed return series (above). They also follow the price series (below) Jan Jul Jan Returns (risk-adj.) Signal (16:32) Signal (8:16) (NB : SPX Index is the index level and is shown as a convenient guide. The implementation uses SP1, the front futures.) Jan Jul Jan-13-1 Signal (16:32) -2 Signal (8:16) SPX Index

9 Some technicalities Two simple things were done that I ve not mentioned, both pertinent to standardisation. 1. Adjusted the returns for volatility obtained by averaging squared returns and imposing a floor (a pretty standard idea) 2. Divided by a normalising constant so that the momentum signal has unit std deviation EMA2 j 0 j 1 ( X n j 1 Now, any asset class will give momentum signals that look the same ) a bit like a common currency for momentum n j X j 0 j 1 ( X 1 ( ) 2 (1 )(1 )(1 n j 2 ) 1/ 2 X n j 1 ) 9

10 Turning a momentum signal into a position Binary rule (already covered): = sgn(mom) Pro: simple, positions bounded Con: sudden change in position at the origin. Con: no positive skew in trading returns ( ) Linear rule: = mom Pro: simple Con: unbounded position Optimal shape shown : = (mom) Bounded position Smooth behaviour Takes risk off when trend v strong Linear combination of different speeds transformed = (z) raw signal 10

11 And here is a typical result USDJPY using 8:16 Notice characteristic pattern of the trading returns. Up fast, down slow Account Apr Apr Apr Apr Apr Apr Apr

12 Skewness seen empirically USDJPY using 8:16 SR ~ but success rate 48% (?!) JY1, 8:16, 50D ret return (per unit stdev) 12

13 Statistical characteristics of momentum systems Skewness of trading returns is positive, under general assumptions This is quite robust to market environment Correct even during fallow periods where money is not being made R J Martin and A Bana, Nonlinear momentum strategies, RISK, Nov 2012 R J Martin and D Zou, Momentum trading: skews me, RISK, Aug 2012 M Potters and J-P Bouchaud, Trend follows lose more often than they gain, Wilmott, Nov 2005 E Acar and S Satchell, Advanced trading rules, Butterworth-Heinemann, 2002 Derivation of second and third moments of trading returns Calculation in special cases Linear and nonlinear systems, i.e. incorporate the response function we saw earlier Hybrid systems which combine positive and negative momentum bets 13

14 Trading returns: moments Period-M trading return, with X the adjusted price of the instrument: First moment is zero (assuming independent returns) Second and third moments which evaluate to returns are uncorrelated but not independent 14

15 Linear systems Signal is a weighted (linear) combination of previous returns Expression for third moment in terms of autocorrelation function where autocorrelation function R is, in terms of the filter weights: 15

16 Trading returns: basic characteristics of skewness Second moment is directly proportional to M Third moment is proportional to M 2 for small M, but to M for large M So skewness is proportional to M +1/2 for small M, but to M 1/2 for large M Sketch for a pure-momentum system (all filter weights positive): skewness return period, M 16

17 Linear systems EMA1, weighting is an exponential where = 1 1/N and N = period or lookback, e.g. N=20 gives =0.95 EMA2, weighting is a difference of exponentials where = 1 1/N and = 1 1/N From these I can calculate R, & then second and third moments, & then the skewness 17

18 Skewness term structure EMA1 and EMA2 Slower oscillators basically stretch in horizontal direction 18

19 Skewness term structure, empirically We have done analysis on basis that mean trading return = 0 However, trend following does work! So mean won t be zero So plot central skew (moments about mean) & non-central skew (moments about 0) CHFUSD and SPX. NB: empirical estimation of skew is very sensitive to outliers 19

20 Hybrid systems 5:10 (short-term) and 20:40 (longer-term) oscillators Red: negative bet on short-term momentum, positive bet on longer-term Green: positive bet on short-term momentum, small negative bet on longer-term 20

21 Nonlinear systems As before, momentum factor is going to be weighted combination of risk-adjusted returns and position θ = / (= signal vol): Third moment and this can be evaluated via where (Z 1,Z 2 ) ~ standard Bivariate Normal with correl 21

22 Choice of For some choices of, the expectation (double-integral) can be evaluated in closed form Two such: 22

23 Results for double-step Narrower separation Lower skewness. Zero separation No skewness (Why?) N=20,40 throughout 23

24 Results for sigmoid Position-limiting reduces skewness 24

25 Results for reverting sigmoid Fast reversion causes negative skewness. Critical ~

26 Optimisation of Can now consider two criteria 1. The classical Sharpe ratio take a selection of futures contracts backtest each one individually and take average SR as the objective function try different shapes 2. Skewness no backtesting of this required! Just compute the skewness analytically and go for highest skewness you can Of course, these two may well conflict 26

27 Optimising the double-step Optimal ~ 0.6: SR not affected, but skewness higher 27

28 Optimisation of a smooth function We can take a linear combination of the two sigmoids 2( ( z) 1), if we choose the same for each, then we have two parameters, and the weight-ratio Use on a universe of different futures contracts and trading speeds ze 2 z 2 / 2 Two criteria, as we wish to maximise Sharpe ratio and Skewness SR: the performance surface rather flat, i.e. hard to be confident about best soln. SR prefers reverting sigmoid but skewness doesn t / 2 ze 2 z 2 so we get a compromise like this: 28

29 Optimisation of a smooth function Linear Sharpe Skew Reverting Linear Saturating 29

30 In the words of Alexander Pope The famous triplet (not couplet, as is often supposed): Nature and nature s laws lay hid in night; God said, Let Newton be! and all was light; Ride good, cut bad, for this we find is right.